As international demand has far exceeded the annual supply of light crude oil in recent years the petroleum industry has increasingly turned to deposits of more intractable crude petroleum. In addition to being geographically remote and in some cases requiring more advanced technologies for their collection, the petroleum products from these fields are often dense and tar-like. These products are known as heavy crude oil, extra heavy crude oil and bitumen (also called asphalt), being much denser than other petroleum products and exceeding that of water. The substances contain relatively little of the paraffinic substances that impart liquid character to the lighter grades of petroleum products and are tar-like in their properties. The viscosities can approach 100,000 centipoise (i.e., 100 Pa·s) because the compositions are a mixed group of complex, highly polymerized resins and aromatic hydrocarbons.
Because the deposits are generally far from refineries the logistics of bringing the product to market are formidable. Before 1980 most produced bitumen was transported by truck, but in a variety of petroleum-rich areas trucking is seasonally restricted due to constraints imposed by climate. Moreover, trucking is relatively inefficient and expensive compared to pipeline transport. Pipeline transport is plausible, but these products are typically too viscous to be pumped readily and or too dense to flow at normal operating temperatures. Consequently the relatively simple conditions required for production, transportation and refining of lighter grades of crude oils are inadequate for this category of product.
One solution was to make the heavier crude products more liquid by diluting them with liquid or low-melting substances. By satisfying pipeline requirements, these products can be transported efficiently and economically to facilities for further processing into conventionally refined petroleum products. The original diluents for this purpose were natural gas condensates (NGC) or their naturally found naphtha component. But because the supply of those is limited refinery products such as refined naphtha and synthetic crude oil (SCO) are also used as diluents.
The amount of diluent required is substantial. A National Energy Board (Canada) study assumed that a standard “dilbit” (i.e., bitumen diluted with NGC or naphtha) contains 33% NGC, and that a natural synbit (i.e., bitumen diluted with SCO) contains 50% SCO. At the refinery dilbit and synbit are typically treated as heavy or medium crudes, respectively. Bitumen shippers use the diluent choice and blend ratio to lower composition costs, increase blend value, and maintain pipeline transportability. Blend ratios may include 25 to 55% diluent by volume, depending on the bitumen properties, diluent characteristics, pipeline specifications, operating conditions, and refinery requirements. Because the blends contain so much diluent it is commonly recovered by distillation from the diluted petroleum stream at the refinery and then reused for instance to convert additional bitumen to dilbit in the field. The amount of diluent needed was so substantial that by 1985 dual pipelines were in use—the first pipeline transported diluent from its source (whether a refinery or a natural source) to the heavy oil fields, and the second transported diluted crude to a refinery.
The available volume and quality of diluent presents ongoing challenges. Natural gas condensates and native naphtha vary in quantity and quality from site to site in their natural deposits, and even within a source they vary. The use of refined diluents provides more consistency but cannibalizes the inventory for fuels and fine chemicals, imposing a substantial opportunity cost.
Gasoline supplies have been expanded by the addition of butane and other light fraction hydrocarbons. For example, butane is often blended with other gasoline components at the refinery, where it is typically added at the trunk line in response to changes in vapor pressure demand. An exemplary refinery blending process is disclosed in Mayer, U.S. Pat. No. 3,751,644. This patent, which is owned by Sun Oil Company, describes a system for automatically adjusting the amount of butane added to a gasoline stream at a petroleum refinery, based on continuous measurements of the Reid vapor pressure of the gasoline downstream from the point of blending. The described process calculates the amount of butane to be blended based on measurements taken downstream of the blending operation, or calculating the blend ratio based on the Reid vapor pressure upstream from the blending operation.
Bajek's U.S. Pat. No. 3,999,959, which is owned by Universal Oil Products Company, also discloses a system for blending butane and gasoline at a petroleum refinery. The Bajek system blends butane with a low-octane gasoline stream and a high-octane gasoline stream, and then analyzes the blended gasoline to measure characteristics such as Reid vapor pressure and vapor to liquid ration.
Efforts at blending butane at a terminal tank farm have also recently been undertaken. As described in U.S. Pat. No. 6,679,302, butane can be blended in-line with a gasoline stream immediately before the gasoline is dispensed on a tanker truck, and after it has been withdrawn from the storage tank. In a preferred process described in this patent, the Reid vapor pressure is measured upstream of the blending operation, and the blend ratio is calculated based on the upstream measurement.
Several methods have been attempted to improve the precision of butane blending and the predictability of Reid vapor pressure in the final gasoline product. The Grabner unit is a substantial advance in this respect. The Grabner unit (manufactured by Grabner International) is a measuring device capable of providing Reid vapor pressure and vapor to liquid ratio data for a gasoline sample typically within 6-11 minutes of introducing the sample to the unit. It has been employed at some refineries to measure the volatility of gasoline consistently, and to blend butane with the gasoline based upon an allowable Reid vapor pressure for the gasoline.
In spite of these advances, there remains a need for a cost-effective method to expand or augment the available supply of diluents to meet market demand. There is also a need to optimize the physical properties of diluent streams for consistent and cost-effective performance.